This type of servo-valve is used by way of example for controlling hydraulic brakes for a vehicle, in particular aircraft brakes.
Pressure regulation servo-valves are known that comprise a body presenting a utilization port, a feed port, and a return port, together with a spool movably mounted in the body so as to put the utilization port into communication either with the feed port or with the return port. The ends of the spool co-operate with the body to define at least one pilot chamber connected to a nozzle that is arranged to open out into a cavity that is connected to the return port. A flapper extending in the cavity and facing the nozzle is mounted to move in controlled manner. The pilot chamber is connected via a constriction to the feed port. Where applicable, the servo-valve may have two pilot chambers exerting opposing forces on the spool, with each of the pilot chambers being connected to a respective nozzle mounted facing each other and having the flapper extending between them.
The controlled movement of the flapper facing the nozzle(s) serves to modulate the pressures that exist in the pilot chambers, and thus to move the spool in order to modulate the utilization pressure.
A drawback of that type of servo-valve is that it presents a leakage flow rate that is considerable. Specifically, hydraulic fluid flows permanently through the nozzle and is redirected towards the return port. This leakage requires the hydraulic fluid tanks to be dimensioned accordingly so as to be capable of ensuring braking of the vehicle even in the event of a failure of hydraulic pressure generation.
A solution to this problem is proposed in Document FR 2 873 828, which consists in connecting the pilot chamber(s) to the utilization port. The utilization port is usually at pressures that are lower than the feed port. Connecting the pilot chamber(s) to the utilization port makes it possible to reduce the pressure that exists on average in the pilot chamber(s), thereby leading to a corresponding reduction in the leakage flow rate via the nozzle(s).
Nevertheless, there exist operating circumstances in which such a position can lead to difficulties in the response of the servo-valve. Specifically, when the brakes are not being used, the utilization pressure is substantially equal to the return pressure, and the pilot force, which is then produced essentially by the spring acting on the spool, can be insufficient to move the spool when the brakes are applied once more, e.g. at low temperature or if the movement of the spool is impeded by an impurity in the hydraulic fluid. Starting is then observed to be difficult, and can sometimes be followed with oscillations in the utilization pressure. The use of a pressure feedback loop for controlling the spool does not help the servo-valve when it is in such a situation.